The present invention relates generally to switching systems for optical signals, and more specifically to a wavelength-division (WD) switching system for optical frequency-shift keying (FSK) signals.
Optical fiber transmission systems, which use optical fibers for transmission lines, are expected to find increasingly extensive use in the future because of their capability of transmitting large quantities of information by virtue of the wide band feature of optical fibers and the immunity of optical fibers from induction noise. Switching systems for use with such optical fiber transmission systems should desirably be optical switching systems capable of switching optical signals in the optical region. In the optical WD switching system, which is one of those optical switching systems, speech or transmission channels are established between the wavelengths of wavelength-multiplexed input and output optical signals, which are switched by converting their wavelengths.
As an example of the optical WD switching system, there is known the configuration described by Nishio et al., "Eight-Channel Wavelength-Division Switching Experiment Using Wide Tuning-Range DFB LD Filters", IEE, ECOC 88, Conference Publication Number 292-Part 2.
In the optical WD switching system described in that literature, however, some optical signals of wavelengths other than the wavelength to be selected by a tunable wavelength filter or a fixed wavelength filter are also transmitted as crosstalk when the tunable or fixed wavelength filter selects optical signals of a certain wavelength from wavelength-multiplexed optical signals. Since the optical signals of different wavelengths are intensity-modulated in conventional optical WD switching systems, the crosstalk adversely affect the optical signals of the wavelength to be selected and invite their deterioration, as described later.